Collisions effect in time-resolved laser induced fluorescence spectroscopy of OH radicals and O atoms in the atmospheric radio frequency plasma jet

Summary form only given. Atmospheric pressure plasma jets (APPJs) generally characterized by high concentration of radicals and low gas temperatures are suitable for many applications. The increasing interest in the application of APPJs heightened the need for quantification of the main active species generated by APPJs. Laser-induced fluorescence (LIF) and two-photon absorption laser-induced fluorescence(TALIF) spectroscopy are the major techniques that have direct access to the ground state populations. They imply two or single photons excitation of the studied species from the ground to an excited state which is following by spontaneous emission (fluorescence) of a photon to an intermediate state. In the atmospheric pressure, quantitative detection of species of interest by LIF and TALIF involves complicated interpretation of the experimental results. The present work uses the timeresolved LIF and TALIF spectroscopy to investigate the temporal behavior of laser excited states in the atmospheric pressure RF plasma, sustained in Ar/0.3%H₂O or 0.3%O₂ mixtures, with a special attention devoted to the proper interpretation of the LIF and TALIF results.The OH radicals was excited from the ground state (X²Π, ν”=0) to the (A²Σ⁺, v´=1) state by laser through P₁(4) transition. The time-resolved LIF spectrum of OH (0 - 0) band (308 nm) demonstrates that the vibrational and rotational relaxation substantially affect the population of OH (A) rotational states. In the APPJs, time and wavelength integrated LIF signal cannot be considered directly proportional to ground density of OH radicals but it is function of VET and RET processes. Both VET and RET have to be considered carefully, especially when LIF signal decay time is used as a parameter for absolute OH density calculations. In case of O atom excitation the working wavelength was fixed at 225.65 nm corresponding to the two- photon excitation energy gap between the fine levels (2p^{4 3}P₂ -> 3p ³P₂) of O. During and after the laser pulse, the time dependent evolution of the plasma spectral emission is successfully recorded by the fast optical emission spectroscopy measurements with 1 a nd 5 ns exposition time. Different from previous researches and theoretical predictions, in addition to the resonant emission of O 844 nm, the TALIF spectrum of the Ar+0.3%O₂ atmospheric plasma is characterized by the spectral irradiance from O 777 nm and Ar lines.The distinct time-resolved behaviors between O (777 nm and 844 nm) and Ar lines emission intens ities can be attributed to the laser direct and indirect excitation process through collision effects.